The present invention relates to the packaging of semiconductor devices, and more particularly to submounts for use in packaging semiconductor devices, such as light emitting diodes.
Light emitting diodes (LEDs) are often packaged within leadframe packages. A leadframe package typically includes a molded plastic body which encapsulates an LED, a lens portion, and thin metal leads connected to the LED and extending outside the plastic body. The metal leads of the leadframe package serve as the conduit to supply the LED with electrical power and, at the same time, may act to draw heat away from the LED. Heat is generated by the LED when power is applied to the LED to produce light. A portion of the leads extends out from the package body for connection to circuits external to the leadframe package.
Some of the heat generated by the LED is dissipated by the plastic package body; however, most of the heat is drawn away from the LED via the metal components of the package. The metal leads are typically very thin and have a small cross section. For this reason, capacity of the metal leads to remove heat from the LED is limited. This limits the amount of power that can be applied to the LED thereby limiting the amount of light that can be generated by the LED.
To increase the capacity of an LED package to dissipate heat, in one LED package design, a heat sink slug is placed under the metal leads within the LED package. The heat sink slug increases the capacity of the LED package to dissipate heat; however, the heat sink slug increases the size, the mass, and the cost of the LED package. Increases in the size, the mass, and the cost are undesirable.
In another LED package design, the leads of the leadframe are extended (in various shapes and configurations) beyond the immediate edge of the LED package body. This increases the surface area of the portions of the leads exposed to the surrounding air. The increased exposed surface area of the extended leads increases the capacity of the LED package to dissipate heat; however, the extended leads increase the size, the mass, and the cost of the LED package.
Another undesirable aspect of the leadframe package design relates to problems associated with thermal expansion of the package. When heat is generated, the LED package experiences thermal expansion. Each of the parts of the LED package has a different coefficient of thermal expansion (CTE). For example, the CTE of the LED, the CTE of the package body, the CTE of the leads, and the CTE of lens are different from each other. For this reason, when heated, each of these parts experience different degrees of thermal expansion resulting in mechanical stresses between the parts of the package thereby adversely affecting its reliability.
To avoid some of the problems associated with leadframe based packages, solid state electronic devices can be mounted on submounts that provide mechanical support, electrical connection, and thermal dissipation, as well as other functionality, for the electronic devices. For example, solid state light sources, such as semiconductor light emitting diodes, can be mounted on submounts as disclosed in U.S. Pre-grant Publication No. 2007/0253209 which is assigned to the assignee of the present invention and which is incorporated herein by reference as if fully set forth herein. The submounts may further be provided in packages that provide protection, color selection, focusing and the like for light emitted by the light emitting device. A solid state light emitting device may be, for example, an organic or inorganic light emitting diode (“LED”). Some packages for light emitting diodes are described in U.S. Pre-grant Publication Nos. 2004/0079957, 2004/0126913, and 2005/0269587 which are assigned to the assignee of the present invention, and which are incorporated herein by reference as if set forth fully herein.
A ceramic-based submount is illustrated in FIG. 1. As shown therein, a submount 5 for mounting an electronic device can be formed by punching or drilling via holes 12 in a substrate of green state alumina 10. As used herein, “substrate” refers to a layer of material that provides mechanical support for an object, such as an electronic device. A submount includes a substrate and therefore provides mechanical support for an electronic device as well, but a submount may also include features that provide electrical connections such as die attach pads, electrical traces, etc., features that dissipate thermal energy, features that provide optical functionality, such as reflectors and/or lenses, and/or other functionality. The via holes 12 can be plated and/or filled with conductive material 14, such as copper or aluminum, and the green state alumina tape 10 and the vias 12, 14 can be co-fired to transform the green state alumina tape 10 into an alumina substrate 10. Contact pads 16, 18, that electrically connect to the vias 12, 14, can be formed on opposite sides of the alumina substrate 10, for example by plating and patterning metal traces. In this manner, electrically and thermally conductive paths can be formed from one side of the substrate to the other.